1. Field of the Invention
The present invention relates to a method and device for timing a pull-in of receiving equipment. For example, a receiver of a modulator-demodulator (MODEM) contains a timing pull-in device which is operative to pull-in an internal clock of the receiving equipment to a timing signal contained in the received signal. The timing pull-in operation is performed in such a manner that the internal clock is synchronized in both phase and frequency with the timing signal. More particularly, the present invention concerns an improved timing pull-in device able to complete the pull-in operation in a very short time.
2. Description of the Related Art
In recent years, it has been common practice to use existing telephone lines for data transmission. Existing telephone lines are usually analog lines in form. Therefore, it is necessary to employ a MODEM which modulates digital data signals into corresponding analog data signals and demodulates the latter signals into the former.
The problem is that the transmission characteristics of the telephone lines to which the MODEM is connected are, in general, not the same, and accordingly, a pull-in operation is essential for each MODEM. A pull-in operation is usually achieved by, first, sending a so-called training signal from a transmitter side to the receiver side every time data transmission is to be started. The internal clock of the MODEM is then synchronized with the timing signal contained in the training signal. After the pull-in operation, the MODEM can start inherent data processing correctly in response to the successive received data signal.
During this pull-in operation, sometimes other pull-in operations are performed in parallel, such as equalization of an automatic equalizer (EQL), automatic again control (AGC), and carrier automatic phase control (CAPC). Through the synchronous pull-in operations, as mentioned previously, the internal clock of the MODEM is synchronized in phase and frequency with the timing signal, i.e., baud rate clock, contained in the received signal at the MODEM. During the reception of the training signal, the pull-in operation regarding the phase is especially important.
In a prior art technique for a timing pull-in operation regarding the phase, a jump in the phase is executed. Namely, the difference in phase between the internal clock and the timing signal is cancelled by the jump in the phase of the internal clock.
The problem in the prior art pull-in technique is that an undesired transient response is produced for a short period immediately after the execution of the jump in phase. The thus produced transient response is inconvenient in that it becomes difficult to increase the data transmission efficiency by shortening the timing signal duration. In practice, such shortening is not possible due to the presence of the undesired transient response within the duration of the training signal, and therefore, that duration is necessarily extended for a period equal to that of the transient response. Then, where the duration of the training signal is required to be shorter than a prescribed value currently used, it is possible that the synchronous pull-in operation cannot be completed within the duration of the training signal due to the presence of the transient response.